The present invention relates to a method for effecting a mold opening/closing step in an injection molding machine having a toggle-type mold-clamping mechanism.
Conventionally, for controlling the speed of a movable plate or of the pressure applied to a movable plate when a mold of an injection molding machine is opened/closed, a method has been employed in which the movable plate speed or the applied pressure is changed depending on the displacement position of the movable plate. A limit switch, potentiometer, or linear encoder has been used to detect the plate displacement position. Especially when the mold opening/closing operation is centrally and remotely controlled, there has been employed a controlling method wherein a displacement position detecting device such as a potentiometer or a linear encoder is used to directly detect the displacement position of the movable plate, and a signal is thereby generated to change the moving plate speed or applied pressure when the detected value agrees with a preset value.
This method is frequently employed not only in direct-pressing type mold-clamping systems, but also in toggle-type mold-clamping systems. Although enhancement of the mold-clamping force just before a mold-clamping operation is achieved in the latter toggle-type mold-clamping operation is achieved in the latter togle-type mold-clamping system, in order to increase the accuracy in detecting foreign matter sandwiched between metal molds, it is possible to provide much higher accuracy in the case where control is carried out by detecting the displacement distance of a movable rod of the mold-clamping cylinder than by detecting the displacement position of the movable plate. That is, since the displacement distance S (stroke) of the cylinder rod is much greater than the displacement distance S' (stroke) of the movable plate, the accuracy is correspondingly higher if control is performed in accordance with the displacement distance of the rod. The ratio between the displacement distances (S/S') is typically larger than about ten, and may be larger than about 20 just before touching the metal mold. The applicant has filed a Japanese patent application (No. 53-31052) concerning such a metal mold protecting device.
In the toggle-type system, there is a correlation between the displacement distance S of the rod and the displacement distance S' of the movable plate. However, as indicated by the graph of FIG. 2, the relationship between the displacement distances S and S' is nonlinear, and therefore the above-mentioned method in which the displacement distance of the movable plate is directly detected to control the speed or the pressure in the mold opening/closing operation has generally been preferred, similar to the direct pressing type mold-clamping system. In a mold opening/closing control method which employs the advantages of a toggle-type mold-clamping system, there has been additionally provided a detecting device such as a limit switch, a contactless switch, or the like, for detecting the displacement distance S of the rod of the mold-clamping cylinder in order to increase the accuracy in detecting foreign matter. However, such a detecting device must be provided not as a central and remotely controlled unit, but as a unit inside a mold-clamping device section of the injection molding machine.
Therefore, because in the conventional injection molding machine having a toggle-type mold-clamping mechanism the displacement distance of the movable plate is directly detected and controlled in order to control the plate speed of or pressure applied to the movable plate in mold opening/closing operations, and further becauase it is necessary to provide another device for detecting the displacement distance of the rod of the mold-clamping cylinder inside the machine, the overall structure of the apparatus is complicated. Furthermore, in the toggle-type injection molding machine, it has been proposed to provide cam means and limit switches for detecting an operational movement of a cross link to control the mold opening/closing operation. However, according to this proposal, since the mold movement and the cross link movement are not kept in a linear relationship, it is difficult to effect the adjustment and setting of the works.